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13 Answers
13

The statement is ambiguous. In what appears to be a "locked room" riddle, we naturally parse the intro and bullets as

Image:

In other words,

four men enter (a room which has no doors etc.). But we could parse it as

Image:

In other words,

four men (who don't have doors etc. with them) enter an empty room. In this interpretation, the only things we know about the room itself are that it is empty and has reinforced walls and ceilings. But it could still have an ordinary complement of openings, which the men took advantage of to enter and can also use to exit. Note that this ambiguity could have been avoided by splitting the intro into two sentences: Four men enter an empty room. The room has 1. No doors and so forth.

First, the men build the large room with extra thicc walls, finishing it up from the outside. Next, they go back in time to before the room was built. They stand in the center of where the room will be, and then travel forward in time to when the room is built. They have just entered the room with no door, windows, entrances, or exists. To leave, they simply go back in time to before the room was built.